Gas phase and slurry processes for the homopolymerization and copolymerization of monomers, especially olefin monomers, are well known in the art. Such processes can be conducted in the gas phase, for example, in a stirred tank or fluidized bed reactor, or in a slurry in a slurry loop reactor. In any of these reactors, particularly in continuous, commercial-scale operation in a fluidized bed reactor, the presence of a thin polymeric coating on the interior surface of the reactor walls, i.e., the surfaces in contact with the polymerization medium, is desirable. Without wishing to be bound by theory, it is believed that the presence of a reactor wall coating inhibits formation of localized areas of electrostatic charge accumulation on the reactor wall surface, a phenomenon that is believed to contribute to the formation of “sheets” of fused resin particles. Sheeting rapidly plugs product discharge systems and/or disrupts fluidization, leading to the need for costly and time-consuming reactor shut-downs.
It is known in the art that reactor wall coatings formed with the aid of certain chromium compounds are particularly effective in reducing charge buildup on reactor walls and impeding sheet formation. U.S. Pat. Nos. 4,532,311 and 4,876,320, for example, disclose methods of reducing sheeting in a fluidized bed reactor by introducing into the reactor a chromium compound, such as a Cr(II) or Cr(III) compound, into the reactor prior to polymerization. The chromium compound can be dissolved in an inert solvent and introduced into the reactor by a pressurized inert gas.
A particular class of useful chromium compounds are chromocenes, such as bis-cyclopentadienyl chromium. Chromocene treatment systems are commonly used to provide reactor wall coatings. Generally, the reactor must be thoroughly cleaned, typically by abrasive such as sand-blasting; subjected to the chromocene treatment; and re-cleaned to remove debris resulting from the treatment. Such treatments, however, require reactor downtimes of typically eight to fourteen days or longer. It has long been desired to develop an alternative method of forming a reactor wall coating that would reduce or eliminate the amount of costly reactor downtime needed to form the coating. It would also be desirable to have reactor wall coatings that further decrease the tendency to form sheets in the reactor during polymerization.